1. Field of the Invention
The present invention relates to a pattern forming method which is applied to an optical lithography step in a semiconductor fabrication process.
2. Description of the Background Art
FIGS. 9 to 12 are sectional views for illustrating a conventional pattern forming method.
Referring to FIG. 9, a photosensitive resist film 2 is formed on a semiconductor substrate 1.
Then, exposure beams 3 are applied to prescribed regions of the resist film 2 to form photosensitized regions 4, as shown in FIG. 10.
Then, the exposed resist film 2 and the semiconductor substrate 1 are heat treated, as shown in FIG. 11.
Then, the heat treated thin film 2 is developed to form a fine pattern, as shown in FIG. 12.
In the conventional method, the resist film 2 is heat treated in advance of development as shown in FIG. 11, for the following reason:
FIG. 13 is a sectional view showing standing waves of exposure beams, which are formed in a photosensitive thin film. In recent years, a semiconductor device has been improved in degree of integration, leading to refinement of a resist pattern therefor. In order to attain such refinement of the resist pattern, exposure beams to be applied to a resist film are prepared in the so-called coherent state, with employment of monochromatic light or light which is composed of a narrow band of wavelengths. Such coherent exposure beams cause a problem of a standing wave effect in the resist film. When coherent exposure beams 3 are applied to a resist film 2 as shown in FIG. 13, multiple reflection is caused between the underlayer for the resist film 2 and the surface thereof. The reflected light components, which progress in opposite directions, overlap with each other to form standing waves 5. Such interference of the exposure beams alternately causes portions having strong exposure intensity, which correspond to loops of the standing waves, and those having weak exposure intensity, which correspond to nodes thereof. Consequently, the portions having strong exposure intensity are alternately stacked with those having weak exposure intensity. When the resist film 2 is of a positive type, the portions having strong exposure intensity are easily developed while those having weak exposure intensity are hardly developed, to cause uneven development. Thus, it is impossible to form a resist pattern in high accuracy.
IEEE Transactions of Electron Devices, Vol. ED-22, No. 7, July 1975, pp. 464-466 discloses a conventional method for solving such a problem of the standing wave effect by heat treating an exposed film in advance of development.
FIG. 14 is a sectional view typically showing a state of heat treating an exposed thin film in accordance with the aforementioned conventional method. Upon such heat treatment, molecules 6 of a photosensitive material which is contained in a resist film 2 thermally move as typically shown in FIG. 14. Therefore, a waving section of a photosensitized region 4 is flattened, so that uneven development is averaged.
However, since the molecules of the photosensitive material thermally move in an isotropic manner upon heat treatment, the film is vibrated in both vertical and transverse directions. While uneven development is reduced by the vertical vibration, the transverse vibration causes deterioration of a fine pattern.
FIG. 15 illustrates concentration distribution of the photosensitive material contained in the thin film, which has been heat treated as shown in FIG. 14. As understood from FIG. 15, the concentration distribution of the photosensitive material is loosely increased and decreased in boundary portions between the photosensitized region and non-photosensitized regions, with no abrupt concentration change. Therefore, a pattern region to be developed is extremely changed depending on concentration of a developing solution or change of the developing time, to reduce accuracy in pattern formation.
Further, the resist film, which is formed of an organic substance, may be thermally denatured by such heat treatment according to the conventional method, to reduce accuracy of the resist pattern.